FIGS. 1 and 2 show a torque rod in accordance with the present invention, and a whole structure is schematically illustrated as a structure of a general torque rod. FIG. 1(A) is a perspective view of the torque rod. FIG. 1(B) is a schematic view showing a state of an engine being supported in a vibration isolating manner. FIG. 2(A) is a plan view of the torque rod, and FIG. 2(B) is a front view thereof.
In the following description of the present invention, each orientation such as front, rear, left, right, upper side and lower side shall be determined based on each direction of the vehicle onto which the engine is loaded. In particular, with respect to forward and rearward directions of a large ring section, the side of a small ring section shall be a forward direction.
A torque rod 10 of general type, as shown in FIG. 1(B) is used as a lower mount which is arranged below an engine 12 in order for providing a connection between a lower part of the engine 12 and a vehicle body.
The engine 12 is arranged transversely, and upper and lower sides thereof are supported by an upper mount 50 and the torque rod 10. The engine 12 is supported by a supporting structure of pendulum type such that both of left and right sides of a principal axis of inertia extending in the vehicle width direction are supported by side mounts (each of which is not shown in the drawing) and the engine 12 which pitches around the principal axis of inertia is supported by the above-referred various kinds of engine mounts.
The torque rod 10 functioning as the lower mount is provided for suppressing the pitching motion. At the time of pitching, the load is inputted into the torque rod so as to pull it forwardly.
This torque rod 10 includes a rod section 40 formed of a long member such as a rod member, a small ring section 20 and a large ring section 30 formed in both ends in the longitudinal direction of the rod section 40. The small ring section 20 is connected to the engine 12, and the large ring section 30 is connected to a vehicle body 14.
As shown in FIG. 1(A), the small ring section 20 is provided with a cylindrical outer member 21 connected to the rod section 40, a cylindrical small ring inner member 22 to be connected to the engine 12, and a small ring elastic member 23 of rubber, etc. providing a connection between the cylindrical outer member and the cylindrical small ring inner member.
The large ring section is provided with a cylindrical large ring outer member 31 connected to the rod section 40, a cylindrical large ring inner member 32 to be connected to the vehicle body 14, and a large ring elastic member 33 of rubber, etc. providing a connection between the cylindrical large ring outer member and the cylindrical large ring inner member.
Herein, letting a center axis of the small ring inner member 22 be C1, a center axis of the large ring inner member 32 be C2, and a center axis of the rod section 40 be C3, these axes intersect at right angles to each other. The center axis C1 shall be arranged in alignment with the left and right direction Y of the vehicle. The axis C2 shall be arranged in alignment with the upward and downward direction Z of the vehicle, and the center axis C3 shall be arranged in alignment with the forward and rearward direction X thereof.
The torque rod 10 is of torsion type in which the small ring inner member 22 of the small ring section 20 and the inner member 32 of the large ring section 30 each are inclined or twisted at 90° about the direction of the center axis C3 of the rod section 40.
Although this torque rod 10, originally, is a vibration isolating device for preventing the vibrations of the engine 12 from being transmitted to the vehicle body 14, it is known that the vehicle body 14 resonates due to the rigid body resonance of the torque rod 10 to thereby produce harsh noises (booming noise).
This is because a predetermined vehicle body resonance frequency range exists in the vehicle body 14 and the vehicle body 14 resonates when the rigid body resonance of the torque rod 10 is produced at this vehicle body resonance frequency range and transmitted from the large ring section 30 to the vehicle body 14.
Therefore, it is desired to reduce the noises on the vehicle body side due to the rigid body resonance of the torque rod 10, and methods for decreasing, in the vehicle body resonance frequency range, the transmission of the frequencies from the large ring section 30 of the torque rod 10 to the vehicle 14 have been examined.
As an example of such methods, there is disclosed a method in which a center of gravity of the torque rod 10 is offset from a straight line connecting the axes of the small ring section 20 and the large ring section 30 by locating a weight in the outer member 31 of the large ring section 30 or by other ways so that the vibrations in the bounce direction and the pitch direction are cancelled by the vibrations in the roll direction (method 1: see a patent reference 1).
Further, it is also known that a peak (see P1, P2 of FIG. 7) of the rigid body resonance in the direction to be transmitted from the large ring section 30 to the vehicle body is varied in such a way as to be shifted from the vehicle body resonance frequency range (method 2).
However, in the above method 1, the resonance peak in a resonance system of the weight has to be adjusted to overlap with the vehicle body resonance frequency range. Namely, since different specifications are prepared for each of vehicle models, the adjustment of the weight must be performed every time a combination between the engine and the vehicle is changed due to differences of the vehicles. Thus, this method is poor in versatility.
Further, in the method 2, as shown in FIG. 7, the peak of the rigid body resonance is merely shifted in parallel from P1 of the vehicle body resonance frequency range to P2 of a low frequency side. Therefore, when the construction and the like of the vehicle are changed due to the differences of the vehicles and the vehicle body resonance frequency range is shifted to the position of P2, the design of torque rod must be changed again such that the resonance peak is shifted accordingly. Thus, this method is also poor in versatility.
To improve this versatility, it is conceivable to make it difficult to transmit the vibrations from the inner member 32 of the large ring section 30 to the vehicle body 14 in the frequency range of 30-200 Hz corresponding to a primary component of frequency by the combustion of the engine. To be concrete, it is known that it is realizable if the transmission characteristics which are a capacity of frequency transmission in the resonance peak are less than 100 N/mm. Herein, the resonance having the transmission characteristics which are less than 100 N/mm shall be referred to as flat characteristics.
The torque rod of such flat characteristics is publicly known. In FIG. 8, a large ring section 130 in the prior art example of such torque rod is shown and is viewed from the direction of a mounting shaft of an inner member 132. This large ring section 130 has a large ring elastic member 133 which elastically connects an outer member 131 and the inner member 132. The inner member 132 is made of metal casting and has a special shape of substantially a triangle.
The inner member 132 is connected to the vehicle body side by a bolt as the mounting shaft. A center of the inner member 132 is designated as a fastening center CT1. Moreover, a center of a spring in the forward and rearward direction of the large ring elastic member 133 is designated as an elastic center CT2. A distance between the fastening center CT1 and the elastic center CT2 is designated as a large ring elastic center distance A (it is positive when the elastic center CT2 is located at the rear of the fastening center CT1).
With this arrangement, the fastening center CT1 comes close to the elastic center CT2 and the large ring elastic center distance is decreased, so that the large ring elastic center distance and a large ring spring ratio R can be comparatively small in such a condition as to fall within a casting zone 61 of FIG. 6 to be referred to later. Therefore, the small transmission characteristics of less than 100 N/mm are realized in combination with a greatly weakened torsion spring of a small ring section to be referred to later.